We have begun a crystallographic study of the repressors from bacteriophage lambda and of the corresponding repressor-operator complexes. We propose to determine, by X-ray diffraction, the structure of the DNA-binding amino-terminal fragment of the lambda repressor and of a co-crystal that contains this protein fragment and an eleven-base-pair synthetic operator fragment. We have crystallized the amino-terminal fragment (which binds specifically to the lambda operators) in a form that is suitable for high-resolution crystallographic study and have determined how the molecules are packed in this crystal. Now we are collecting native data and searching for isomorphous derivatives. We have also grown crystals that contain both the amino-terminal fragment of repressor and the synthetic operator fragment. Our best co-crystals diffract to approximately 3.5 A resolution in one direction and 6 A resolution in the other directions. We will test other crystallization conditions and will use other synthetic operator fragments. We will also try to grow co-crystals that contain intact lambda repressor and co-crystals that contain intact cro protein.